Hmmm.... I'm really not sure about trying to glue it shut. It's also a pretty big crack - while epoxy would be the first choice trying to get it down in there will be hard. If you do use epoxy, warming a slow cure epoxy would help. Definitely do not use a fast cure epoxy. Brownell's Acraglas is always my first choice. Not all epoxies are created equal. In general, the faster an epoxy cures, the worse it holds up over time and with shocks.
With that big of a split horn handle, there's a lot of tension in there now and I think I would fill it vs. clamping shut given how wide open it is. You can use a bunch of clamps or rope. Epoxy cures chemically and is why you mix two parts. CA (superglue) cures based on moisture in the air. In general, you have a full cure in a couple of hours if I recall right. Dry does not equal cured with epoxy or CA.
I'd put CA (super glue) in the crack and use 220 grit sand paper to put hoof material in with the glue. It will clog the sand paper fast so you will use a lot of it. Wear a breathing mask and either goggles or good ventilation as the CA glue will really irritate your eyes. There are versions of CA that can fill gaps but most soak into a crack and then need filler added.
Once the crack is full, sand up to 1000 or 2000 grit and then apply the conditioner. I usually go 220, 320, 400, 600, 800 and then I may go to 0000 steel wool depending on the look I want. I probably write this differently each time depending on the project I just did. I can tell you that if you go up to 3200 and apply conditioner then you get a nice rich black obsidian look.
Does this help?